There can hardly be a more serious charge against a prime minister than that of taking a country to war before peaceful alternatives have been exhausted, undermining the authority of the UN security council, leaving Britain more at risk of a terror attack, and helping to provoke a conflict that led to the deaths of 179 British troops and of at least 150,000 Iraqis.

The Chilcot report is an unprecedented, devastating indictment of how a prime minister was allowed to make decisions by discarding all pretence at cabinet government, subverting the intelligence agencies, and making exaggerated claims about threats to Britain’s national security.

This is not the whitewash sceptics had previously predicted the inquiry by the former senior Whitehall mandarin would be. After years of heated exchanges, Chilcot managed to prise out of successive cabinet secretaries the right to declassify some of the most sensitive documents – sensitive not because of Britain’s security but because they show how private talks and notes between friendly leaders can prevent the public from knowing what is being promised behind their backs on matters of life and death.

The report includes a telling document, called simply “note on Iraq”. It reveals that Tony Blair told George W Bush on 28 July 2002, eight months before the invasion of Iraq: “I will be with you, whatever.”

He added: “But this is the moment to assess bluntly the difficulties. The planning on this and the strategy are the toughest yet. This is not Kosovo. This is not Afghanistan. It is not even the Gulf war.

“The military part of this is hazardous but I will concentrate mainly on the political context for success.”

Yet Chilcot hints at, though ducks, even the question of what political pressure was put on the attorney general, Peter Goldsmith, before he changed his advice. In carefully chosen words, the report says: “We have, however, concluded that the circumstances in which it was decided that there was a legal basis for UK military action were far from satisfactory”.

The Chilcot inquiry had heard how on 11 February 2003, less than a month before the invasion, Lord Goldsmith met John Bellinger, a legal adviser in the White House. Bellinger reportedly later recounted: “We had trouble with your attorney. We got him there eventually.” There was no change of fact to allow Goldsmith’s last-minute change to take place, Philippe Sands QC, a professor of international law, said earlier this week.

The excoriating report does leave hanging a really big question. How should, how could, those criticised in it, not only Blair, be called properly to account for their actions?

Chilcot and his team do not shy away from casting their criticism far and wide. Individuals under fire include Sir Richard Dearlove, then head of MI6, and Sir John Scarlett, chairman of Whitehall’s joint intelligence committee, who was to succeed Dearlove a year after the invasion. In a withering passage, the report says the misleading Iraq weapons dossier has produced “a damaging legacy, including undermining trust and confidence in government statements, particularly those that rely on intelligence that cannot be independently verified”.

Relatives of some of the British service personnel killed in the Iraq war. Photograph: Chris J Ratcliffe/Getty Images

Senior military figures giving evidence to the inquiry blamed their political masters for not allowing them enough time to prepare for war, and not giving them enough money to get proper equipment. Chilcot makes it clear that the armed forces should have “spoken truth to power”, should have revised their plans and stopped to think – notably when Blair agreed to send thousands of British troops, of whom there was already a shortage in southern Iraq, to Afghanistan.

But this scattering of criticism should not be allowed to soften the criticism of Blair himself. Chilcot put it bluntly in his statement on Wednesday: “Ministers were aware of the inadequacy of US plans … Mr Blair … did not ensure that there was a flexible, realistic and fully resourced plan that integrated UK military and civilian contributions, and addressed the known risks.”

Blair went ahead with the hopelessly unplanned adventure even though there was a risk of conflict between rival groups in Iraq, as one of his notes to Bush released on Wednesday reveals. “The biggest risk we face”, he told the president in January 2003, two months before the invasion, “is internecine fighting between all the rival groups, religions, tribes etc, in Iraq when the military strike destabilises the regime”.

The invasion of Iraq has been described as Britain’s worst foreign policy disaster since Suez. It was much worse. The 1956 Suez crisis marked the death of Britain’s imperial presumptions and demonstrated how it had to bow to Washington after the US failed to stop the run on sterling.

The decision to deploy troops to Egypt (after President Gamal Abdel Nasser decided to nationalise the Suez canal) did not in itself have a lasting impact on Britain’s role, or even reputation, in the Middle East. The invasion of Iraq has had a profound effect on Britain’s reputation – as some in MI6 and the Foreign Office warned at the time to no avail – as well as, of course, on the stability of the region. The legacy of the invasion is evident every day in the violence in Iraq and Syria.

Bush and Blair at a joint press conference in 2005. Photograph: Jason Reed/Reuters

But like Suez, the invasion of Iraq did touch directly on the relations between the UK and the US. Some senior Whitehall figures told me at the time that it was virtually impossible for a British prime minister not to succumb to the wishes of a US president on a matter of such importance to Washington. Others point to Harold Wilson’s refusal to enter the Vietnam war.

Chilcot made his view clear on Wednesday. “There are many lessons set out in the report”, he said. “Some are about the management of relations with allies, especially the US. Mr Blair overestimated his ability to influence US decisions on Iraq. The UK’s relationship with the US has proved strong enough over time to bear the weight of honest disagreement. It does not require unconditional support where our interests or judgments differ.”

Day after day in the run-up to the invasion, MI6 officers (many of whom were dismayed by the attitude of their boss, Dearlove) and officials in the Foreign Office gritted their teeth as they prepared for the worst. They were either shut out of Downing Street or their minister’s private offices, or allowed themselves to be. Risks were taken and voices were silenced, with consequences going well beyond Iraq – not only as far as future British military operations abroad are concerned but for Britain’s role in the world, a question made even more urgent by Brexit.

• Richard Norton-Taylor writes for the Guardian on defence and security matters and previously was the paper’s security editor, including during the years running up to, and after, the 2003 invasion of Iraq. He has written several plays based on transcripts of public inquiries including Chilcot, performed recently at the Lowry theatre, Salford, and the Battersea Arts Centre in London